Toss the chicken thighs with the quarter cup of cornstarch and let sit while you mix the sauce ingredients.
Add the rice vinegar, soy sauce, hoisin sauce, water, sugar and tablespoon of cornstarch in a small bowl and whisk together. Add the chicken to a pan with the oil and fry until crispy.
Drain all but a tablespoon of the oil and add the chili flakes, ginger and garlic.
Cook until you smell the garlic (about 30 seconds).
Add in the chicken and toss, then add in the sauce. Stir for about 30 seconds until thickened.
Serve immediately.